1. Field of the Invention
The present invention generally relates to injection molding machines for forming plastic products. More specifically, this invention relates to an injection molding machine having a pressure assist nozzle which operates to compress the molten plastic within the mold and shut off the flow of molten plastic to the injection mold.
2. Description of the Prior Art
Injection molding machines are widely used to produce plastic products having geometries with varying degrees of complexity. Injection molding machines are typically composed of an extruder, an injection manifold which is provided with molten plastic by the extruder and one or more injection molds which are fed the molten plastic, or melt, by the injection manifold. The extruder is typically provided with solid plastic shot or pellets through a gravity-fed hopper or the like. The plastic shot is plasticized within the extruder by way of heating elements within the extruder's walls. The extruder, through a plunging action or a rotating screw thread action or a combination of both, then operates to provide the injection manifold with a desired quantity of the melt. U. S. Pat. No. 3,335,457, typifies a common form of extruder which uses a reciprocating screw to deliver the melt to the manifold.
From the above it can be readily seen that it is desirable to achieve sufficient mold fill with the melt such that voids are prevented within the mold. Generally this requires an external pressure or displacement source which insures that the melt is sufficiently compresses with the mold throughout the fill and cooling processes. In addition, by providing pressure within the mold and manifold, voids created by shrinkage during cooling are also avoided.
In many typical injection molding processes, the extruder performs the function of compressing the melt within the mold. However, a disadvantage of this method is that the use of the extruder to insure sufficient fill of the mold prevents the extruder from preparing for the next plasticizing cycle. Therefore, it has been suggested in the art to provide an additional mechanism between the extruder and the mold to perform the compression step. An example of such a device is illustrated in U.S. Pat. No. Re. 28,721 to Farrell. Farrell teaches the use of separate shut off and pressurizing mechanisms which operate sequentially by way of a three way valve. The three way valve closes a passage between the extruder and the mold to stop the flow of melt, and then subsequently puts the mold in communication with an accumulator which pressurizes the melt within the mold. A disadvantage of the device taught by Farrell is the requirement for two separate mechanisms to accomplish the shut off and pressurization operations, resulting in more parts and a less compact injection molding machine.
In addition to the above, it is also important for economical reasons to supply to the molds the necessary quantity of melt to sufficiently fill the molds, and no more. When an excessive amount of melt is provided to the molds, flash is formed between the mold halves which must be removed for aesthetic and/or functional reasons. When an insufficient amount of melt is provided to the molds, voids form within the molds, again negatively affecting the final product aesthetically or functionally. The development of voids is aggravated because the melt shrinks as it cools within the mold.
For purposes of minimizing waste which occurs as mold flash between the mold halves, it is important in injection molding processes to shut off the flow of the melt to prevent excessive melt from being introduced into the mold. Consequently, a device for stopping the flow from the extruder to the mold is typically used to allow a predetermined quantity of melt to flow from the extruder. The quantity of melt is metered to avoid excessive melt from being injected into the mold and, accordingly, mold flash that accounts for waste is minimized.
Variations of this basic concept are embodied in U.S. Pat. No. 4,41,807 to York and U.S. Pat. No. 2,862,241 to DeMattia. York provides a shut off valve which is especially adapted to prevent weld lines from forming within the melt which remains within the injection molding machine between molding operations. DeMattia goes a step further by teaching the use of a spring loaded plunger which, upon the closing of a shut off valve, retracts to accommodate any excess melt, thereby reducing mold flash. Another example of a shut off valve used in the prior art is illustrated in U.S. Pat. No. 4,155,969 to Hendry which uses a shut off valve in the operation of a two step injection process.
From the above discussion it can be readily appreciated that the prior art does not disclose an injection molding machine which provides the advantages of both the shut off operation and the pressurizing operation all within one mechanism. In particular, the prior art does not provide a valve mechanism that acts to close the flow of the melt to the mold while also, by means of its operation, acts to maintain pressure within the mold to prevent voids from forming.
Accordingly, what is needed is an injection molding machine that is adapted to inject a predetermined quantity of melt into a mold, maintain pressure in the mold in a manner that prevents voids from forming within the melt, while also reducing extruder cycle time. In addition, what is needed is such a machine which provides both operations within one valve body to reduce the number of components required to perform the operation and to simplify manufacture and assembly of such an injection molding machine.